This is a randomized, open-label, parallel group study to determine the optimal dose of CART-19 cells (autologous T cells expressing CD19 chimeric antigen receptors expressing tandem TCR Zeta and 4-1 BB co-stimulatory domains) of the two dose levels being assessed (1-5x10e8 vs. 1-5x10e7 CART-19 cells). This trial will be conducted in two stages.